The field of the present invention relates to data reading systems and particularly to an optical system having a multiple function outgoing beam. The invention may be especially suitable for use with a symbol scanning system for reading bar codes such as those found on consumer products, for example the UPC code. The invention is suitable for stationary or handheld scanners.
Bar code scanners, as any optical system, depend upon focused optics for effective and accurate performance. Typical bar code scanners employ a source of coherent light from a laser or laser diode with the light scanned in various directions across a window. Other scanners using non-coherent light sources have also been suggested such as disclosed in U.S. Pat. No. 4,335,302.
In a detection system such as a bar code scanning device employing a focusing lens, a light source (such as a laser, laser diode, or non-coherent light source, e.g. light emitting diode) emits light which passes through and is focused by the focusing lens. The object containing the bar code is passed through the focused beam and if the beam spot striking the bar code is of sufficiently small size and shape, reflected light from the bar code may be detected resulting in a successful scan.
As will be described herein, a light beam such as a that produced by a laser diode is typically not a discrete point but is focused to what may be referred to as a "waist" which is the position along the beam axis where the "cone" of light from the light source reaches a minimum spot size in a given direction, typically in a direction parallel to the direction of spot motion.
A problem arises when the bar code or object being scanned does not fall sufficiently close to the waist, that is when the spot size or waist diameter is too large (generally in the direction parallel to the direction of spot motion) to successively read a symbol. By way of example referring to a handheld scanner in a market checkout application, a product bearing the UPC bar code is passed at a certain distance in front of the window of the checkout scanner. The checkout scanner is designed with its waist of a given diameter positioned at a certain distance from the window where the bar code is expected to pass. The checkout clerk must become familiar with the proper distance to pass the object in front of the window, that is the bar code must pass sufficiently close to the scanner focal point or waist (i.e. within its depth of field) in order to achieve a successful scan.
A typical focusing lens must ordinarily be manufactured to relatively exacting specifications in order to provide the desired focusing properties. Assorted manufacturing variances, in both the focusing lens and other elements, may introduce undesirable effects into the optical beam. Temperature effects may also introduce variations into the optical properties. Moreover in some applications, it may be desirable for the scanning device to possess a plurality of operations, for example the ability to function over a range of distances or provide an aiming spot.
There have been several suggestions on how to increase the depth of field available for a particular scanner. In one system, a focusing lens is designed with an axially movable lens element (such as a zoom lens) to permit changing the focusing power to change the depth of field. Such systems require complicated mechanical lens adjustment and/or may require the user to manually make focusing adjustments. It is desirable to eliminate the need for focus adjustment either by mirror or lens adjustment and be able to achieve a wide range or variable range of focal distances.